This invention relates to a process and apparatus for the low temperature fractionation of a gas, e.g., an ammonia synthesis waste gas into hydrogen, nitrogen, argon and methane, especially to a system comprising two successive fractionating stages wherein a nitrogen refrigeration cycle is employed for supplying reboiler heat for each of the stages and liquid nitrogen as well.
In the production of ammonia synthesis gas by steam reforming, there is obtained a waste gas rich in argon and methane, besides being rich in hydrogen and nitrogen. According to a conventional method [Winnacker-Kuchler, Chem. Technologie 2:494 (1969)], this waste gas is fractionated in a low-temperature process wherein, on the one hand, the hydrogen is recovered, and on the other hand, pure argon is produced. Fractionation is conducted in two successive separating stages. A nitrogen refrigeration cycle is provided for producing the low temperatures required for the separation of the components. Nitrogen from the head of the second separating stage as well as from a storage tank is compressed to 150-200 bar and then cooled. A partial stream of the cooled compressed nitrogen is engine-expanded and utilized for heating the sump of the second separating stage. Another partial stream of the cooled compressed nitrogen is further cooled by heat exchange with the uncompressed nitrogen and utilized to heat the sump of the first separating stage. The two partial streams are subsequently expanded, in partially liquefied form, into a storage tank. Liquid nitrogen is withdrawn from the storage tank as scrubbing liquid for the second separating stage and for cooling the head of the first separating stage. A portion of the nitrogen which has remained in the gaseous phase, together with nitrogen from the head of the second separating stage, is heated in heat exchange with synthesis waste gas feed, while another portion of the nitrogen which has remained in the gaseous phase is heated in heat exchange with nitrogen for the heating of the first separating stage and is recompressed. A portion of the liquefied nitrogen, after revaporization, is recompressed together with a portion of the gaseous nitrogen. Excess nitrogen is withdrawn from the plant after heat exchange with the synthesis waste gas feed.
Although this process is advantageous insofar as it permits the recovery of hydrogen and argon, it is very expensive with respect to the equipment required owing to the high pressures required in the nitrogen refrigeration cycle. Pressures on the order of 150 to 200 bar involve the use of high pressure compressors and heat exchangers which are expensive, trouble-prone, and difficult to service.